The present invention relates to 4-pole electric motors, more particularly to 4-pole DC motors, wherein separable back-up cores are provided at the ends of a cross-shaped core of the armature, resulting in elimination of the dead point for start-up, promotion of the efficiency of coil winding, and increase in quantity of windings applicable accompanied with enhanced power efficiency.
In motors which have a permanent-magnet ring surrounding an armature with electromagnetic poles having coil-wound cores, with either the armature or the ring serving as the rotor with the other as the stator, turning off the current to the electromagnetic poles will stop the rotor stably at the position of lowest magnetic resistance, depending on the relative positional relation between rotor and stator.
In restarting the motor with the rotor in such a position, the so-called start-up dead point is encountered where the ring poles and electromagnetic poles are in such a positional relation that there are diametrical N--N and S--S orientations with respect to the center of rotary shaft, which is incapable of generating rotary force.
A means for avoiding the start-up dead point has been adopted in which the core itself is deformed in shape. However, this means has disadvantages that projections, etc. disturb the winding job and that the quantity of coil windings on the armature is reduced to lower the power efficiency.